Ara Vartabedian

Notched tensile creep testing of ceramics

The use of axisymmetric tensile creep specimens containing semicircular notches is shown to provide important data which complement results of the common smooth tensile creep test. The notch provides a stress and strain concentration which can represent the non-uniform state of actual components in service. Finite element analysis confirms that the initial concentration of stress redistributes during creep to yield a more uniformly loaded cross-section. Additionally, it is shown that the stress follows a highly non-proportional loading path during creep. The first experiments of notched tensile creep of a ceramic (silicon nitride) are examined in the light of finite element simulations of the transient deformation. The implications of notched tensile creep are discussed as an important test for the evaluation of constitutive models.

Development and Testing of High Temperature Silicon Nitride Microturbine Rotors

Development and testing of gas turbine quality high temperature silicon nitride (NT154) components will be described. An advanced CNC green (using pressed powder blanks prior to densification) machining based complex shape forming methodology has been developed and successfully deployed to fabricate gas turbine quality rotor, vane ring and shroud components to net shape with high yield and required dimensional tolerances. Utilizing a systems approach involving green blank properties, type of cutting tools and machining parameters, the process has been optimized to achieve required as machined surface finish, dimensional control and part integrity. Integral bladed micro turbine rotors (IBR) and vane rings have been fabricated with dimensional control within 100 microns, surface finish within 1–2 microns. IBRs so formed have been successfully spin tested at room temperature at 40% above designed speed approaching maximum stress in the vicinity of 400 MPa.Copyright